Though it's not a subject most of us want to discuss, the fact is that bad breath is a problem that affects one in four people.

It's typically caused by sulphur- emitting substances in the mouth, which trigger the odour. These occur when cells start to die and decay as part of the natural process of cell renewal or they can also be produced by certain bacteria that live in the mouth.

How much breath smells depends on the level of odour-causing bacteria or decaying cells in the mouth.

This reflects how well you clean your teeth and the state of your oral health.

Though it's not a subject most of us want to discuss, the fact is that bad breath is a problem that affects one in four people

Gum disease is the most common cause of chronic bad breath: it leads to inflammation of the gums, which makes them swell and separate from the teeth, creating gaps, called pockets, that trap bacteria and decaying cells and increases the rate at which cells die.

'The poorer your dental hygiene or the more crowded/crossed and hard to clean between your teeth are, the more likely it is that you'll suffer problems,' says Dr Mervyn Druian, a dentist and halitosis specialist at the London Centre for Cosmetic Dentistry.

However, you don't have to have gum disease — or poor dental hygiene — to have bad breath...

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The most common oral cause of bad breath aside from gum disease is an overgrowth of odour-causing bacteria on the tongue.

It's not clear if this is related to diet, smoking or other issues, but the most common area for the bacteria to build up is right at the back of the tongue.

It's an area most of us don't include in our daily tooth cleaning regimen, according to Professor Robert Allaker and Dr Abish Stephen, oral microbiologists at the Institute of Dentistry at Queen Mary University London, and two of the UK's leading researchers into breath odour.

'Cleaning the tongue gently with a toothbrush can help reduce odour,' says Professor Allaker.

The most common area for the bacteria to build up is right at the back of the tongue. This is an area most of us don't include in our daily tooth cleaning regimen

'Tongue scrapers can be used, but there's no evidence they are any more beneficial than brushing gently.

'Mouthwash also helps as part of a daily routine as it inhibits the growth of bacteria that cause odour and neutralises bad smelling compounds.'

Another potential treatment is oral probiotics — gum or supplements that introduce beneficial bacteria to the mouth.

They crowd out odour-forming bugs, but Dr Stephen says: 'These show promising results, but we don't know if there are any long-term benefits of changing the bacteria this way.'

ARE YOU TALKING TOO MUCH?

Anything that dehydrates the mouth will increase the risk of bad breath as saliva helps wash away odour-causing bacteria or cells.

This is why odour is worse in the morning when saliva flow is reduced and is also more common in people who breathe mostly through the mouth.

'Talking a lot is another factor as it exposes the mouth to air — drying out saliva,' says Dr Druian.

Dr Mervyn Druian says anything that dehydrates the mouth will increase the risk of bad breath, including talking too much

'So those in vocal professions, such as teachers, lawyers and people working in call centres, are particularly prone.

'The solution is quite simple — sip water throughout the day to keep the mouth moist.'

Reduced saliva flow is also more common as we get older and as such bad breath tends to worsen with age.

New research from the Medical University of Sofia in Bulgaria has found stress hormones cause bacteria involved with bad breath to colonise the mouth at a faster rate.

So, cleaning, flossing and using mouthwash are important if you are under pressure.

New research from the Medical University of Sofia in Bulgaria has found stress hormones cause bacteria involved with bad breath to colonise the mouth at a faster rate

YOU HAVE BEEN SKIPPING MEALS

Because it stimulates saliva production and moves food and bacteria out of the mouth, the simple act of eating helps reduce bad breath.

Studies by the Dental Clinic of the University of Bern in Switzerland found breath odour was reduced for an average of 2½ hours after eating — longer if the meal contained fibre, which acts almost like a toothbrush.

So not eating regularly could lead to odour.

'Fasting plans or high-protein/low-carb diets that switch the body into a state called ketosis where it burns fat for fuel can trigger bad breath,' says Dr Druian.

Not eating regularly could lead to odour. Because it stimulates saliva production and moves food and bacteria out of the mouth, the simple act of eating helps reduce bad breath

The odour comes from substances called ketones, which are produced when we burn fat.

This breath smell, often described as sweet or smelling like nail varnish remover, won't disappear unless you stop the diet. Drinking more water or peppermint tea and chewing parsley can help disguise it.

STONES IN YOUR MOUTH

Your tonsils are not smooth — they have dips and pits in them known as crypts.

'If food or other debris get caught in these crypts it creates an environment where the bacteria that causes bad breath can thrive,' says James Tysome, an ENT surgeon at Addenbrooke's in Cambridge and Spire Cambridge Lea Hospitals.

This is particularly likely if the debris hardens, a problem known as tonsil stones.

Your tonsils are not smooth — they have dips and pits in them known as crypts. If food gets caught in these crypts it creates an environment where bacteria can thrive

A 2008 Brazilian study found 75 per cent of people with high levels of sulphur compounds in the breath also suffered tonsil stones.

Salt gargles and antibiotics can reduce tonsil stones by killing bacteria. 'But the only cure if you develop them regularly is removing the tonsils,' says Mr Tysome.

'However, this is only likely to be done if they are severely impacting on your quality of life.'

IT'S THE STOMACH ULCER BACTERIA

Helicobacter pylori is bacteria found in the stomach of 40 per cent of the population.

It's most commonly associated with the development of stomach ulcers, but it has also been linked to bad breath.

In fact, a 2015 study from the Peninsula School of Medicine and Dentistry in Plymouth found that eradicating helicobacter pylori with antibiotics dramatically reduced halitosis in people who had the bacteria and bad breath.

A bacteria most commonly associated with the development of stomach ulcers has also been linked to bad breath

'This bacteria produces sulphur compounds and ammonia that alter the odour of the breath,' says Professor Allaker. But if it lives in the stomach, how can odour end up in the breath?

The answer is acid reflux — if the reflux contains helicobacter pylori this can change the smell of the breath.

If bad breath is associated with heartburn, a hoarse voice or food regurgitation, ask your doctor to test for this bacteria.

YOUR SINUSES ARE BLOCKED

'I get a lot of people coming to see me complaining of bad breath and upon examination I find the problem is that they have something wrong with their nose,' says Mr Tysome.

'They're suffering chronic sinusitis — an inflammation of the sinuses, which are the air-filled cavities behind the cheekbones and forehead.'

When you have sinusitis, mucus, which has high levels of proteins that feed the odour-causing bacteria, drips down the nose into the throat

Mucus, which has high levels of proteins that feed the odour-causing bacteria, drips down the nose into the throat.

A steroid-based nasal spray reduces inflammation and the symptoms that come with it.

OR IT COULD BE DIABETES

Bad breath may be triggered by chemicals in the blood.

'We call this blood-borne halitosis and while it's not a common cause of bad breath it should be considered,' says Dr Saliha Saad, an oral microbiologist at the University of West England.

It's most commonly related to diseases where toxins build up in the blood and odours associated with these are excreted through sweat or breath.

Bad breath is most commonly related to diseases where toxins build up in the blood and odours associated with these are excreted through breath, such as diabetes

Diabetes gives the breath a sweet, sickly scent (from ketones building up in the blood), liver disease causes a musty odour and kidney problems result in a fishy/ammonia-like smell.

CHECK YOUR MEDICINES

Certain medications can also be associated with bad breath — most commonly because they dry the mouth.

These can include anti-depressants, anti-histamines and blood pressure medication.

Certain medications can also be associated with bad breath — most commonly because they dry the mouth

If your problem started when you began a new drug, ask your GP if dry mouth is a known side-effect — they may prescribe an alternative medication.

YOU'RE SIMPLY IMAGINING IT

When researchers in Belgium analysed the cause of bad breath in 2000 patients attending a bad breath clinic, they found 16 per cent didn't have bad breath at all — they were imagining it.

When researchers in Belgium analysed the cause of bad breath in 2000 patients attending a bad breath clinic, they found 16 per cent were imagining it

'It is called halitophobia and people with it often cover their mouth when they speak, avoid getting close to people and are fixated with oral hygiene because they are so convinced their breath smells,' says Professor Allaker.

The first stage in diagnosis is an independent assessment of breath odour (not available on the NHS).